Wednesday, July 31, 2013

With only today left, July 2013 is in a virtual dead heat with July 2007 for the hottest month in the history of Salt Lake City. The average temperature for each month is 84.0ºF.

Source: http://http://xmacis.rcc-acis.org

I don't know if official records are based on the average to the nearest tenth of a degree (at some point, it's not worth slicing the onion any thinner), but let's assume that they are. That means we need today's average temperature to be 84ºF or greater to tie or break the record.

Helping a great deal is the overnight minimum, which was a balmy 75ºF. That means we need a high of 93ºF to at least tie the record (assuming it is based on the average temperature to the nearest tenth of a degree). The NWS forecast calls for a high of 98ºF.

Of course, this all assumes that the unofficial numbers I'm using are in line with those at the NWS. Let's hope they are. I like seeing records happen.

Tuesday, July 30, 2013

After yesterday's cool afternoon, we've slipped back into a virtual tie with 2007 for the hottest July on record.

Source: http://xmacis.rcc-acis.org

Using this morning's minimum and the National Weather Service forecasts for maximum and minimum temperatures through tomorrow puts us just behind 2007 at the end of the month. Don't let it slip away!

Fortunately, we're still well in the lead for the hottest June–July combo. Our suffering will not go to waste.

Source: http://xmacis.rcc-acis.org

I should add that data above comes from a reliable source (the NOAA Regional Climate Centers), but is unofficial. Thus, given the narrowness of this race, we'll ultimately need to see what the National Weather Service has to say at the end of the month.

Monday, July 29, 2013

July is nearing an end and the only question at this point is how many records will fall at the Salt Lake City Airport. Some are going down to the wire.

The record for the hottest combined June–July period (based on average temperature) looks likely to fall. Despite the recent cooling (in terms of maximum temperatures), we are running about .75º ahead of the next closest year, 2007.

Source: http://xmacis.rcc-acis.org

Our string of consecutive days with above average minimum temperatures also remains intact. The last day with a below average minimum temperature was June 23, 35 days ago.

Source: NWS

I don't know what the record is for consecutive days above average—someone will need to do some sleuthing—but I think this is an impressive run. Forecast minimum temperatures for the next few days suggest we will make it until the end of July, but one rogue thunderstorm and it's over. I see it as likely but not a lock that we make it until then.

Going down to the wire is the record for the hottest July (based on average temperature). We currently sit just 0.2ºF ahead of 2007.

Source: http://xmacis.rcc-acis.org

A day or series of days adding up to 6ºF below average would erase that lead. Right now, it appears we will eke it out, but keep your fingers crossed. I've already printed up hundreds of T-shirts in anticipation of a record...

Sunday, July 28, 2013

In Colorado, where thunderstorms are pretty much a daily occurrence during the summer, the rule of thumb is to be back down to timberline by 1 PM. Here in northern Utah, thunderstorms are not always a daily occurrence, but that rule of thumb turned out to be quite useful today.

Given the forecast, I opted for a mid-morning hike up Patsey Marley. I awoke to just a few shallow cumulus clouds over the mountains, and kept an eye on things during the ascent, which proved uneventful. On top at about 11:45, the sky was filled with cumulus, which were growing in depth, but hadn't yet become deep enough to glaciate (i.e., develop ice crystals), which is essential for the electrification. Rather than push it, I enjoyed lunch on top and decided to head down.

Subsequently, surface heating continued to destabilize the atmosphere, resulting in increasingly deep convective clouds and eventually the development of thunderstorms over the Oquirrh Mountains and portions of the Salt Lake Valley. The growth of the convective clouds was well captured by George Wilkerson's Cove Cam, which looks westward toward the Oquirrhs from Olympus Cove.

The spotty nature of convection is such that you can get lucky and not be in it, but it's better to finish early and enjoy a cold drink at home than to be cowering on a high ridge. In addition, I can't sprint downhill to safety as easily as I could a few years ago!

Friday, July 26, 2013

In the previous post (Intricacies of Local Temperature Trends, Part I), we showed that average summertime temperatures have increased in recent decades (since 1948), but also that minimum temperatures are increasing faster than maximum temperatures, resulting in a decrease in what meteorologists call the diurnal (daily) temperature range (DTR).

There are potentially a number of mechanisms contributing to these trends. These mechanisms include:

1. Global warming. It is well documented that average global surface temperatures have increased in recent decades and that this has been accompanied by a decrease in the DTR at many stations.

During this period, the planet's energy balance has been modified by changes in greenhouse gas concentrations, aerosol (i.e., pollution) concentrations, and other factors. Increasing temperatures and a decreasing DTR are consistent with what we expect from the net influence of these changes, but trends at any given location such as the Salt Lake City airport are also going to be influenced by regional climate variability and related changes in the large-scale atmospheric circulation, cloud cover, and aerosol concentrations.

Thus, we can conclude that trends at the Salt Lake City airport are consistent with expectations from global warming, but the size of the contribution remains undetermined. I suspect it is a significant, but that other factors noted below are also important.

2. Urbanization. The urban heat island typically acts to increase temperatures and reduce the DTR in developed areas compared to rural or undeveloped areas. The urban heat island of the Salt Lake Valley is somewhat unique, however, since in addition to adding buildings and the like, we have replaced the natural, semi-arid landscape with an urban forest (i.e., grass, trees, etc.) that is heavily irrigated.

Urban tree canopy in 2006. Source: Lowry (2010)

Thus, although many urban areas are characterized by less evaporation and transpiration compared to the surrounding rural or undeveloped region, evaporation and transpiration are greater in the Salt Lake Valley than they would be with a natural landscape, resulting in a cooling oasis effect.

So, we have traditional heat island effects that would contribute to higher temperatures (maximum and minimum) and a reduced DTR compared to an undeveloped Salt Lake Valley, but this is countered somewhat by the oasis effect. In the net, I'm not sure what wins for the maximum temperature, but it is likely that urbanization in the Salt Lake Valley would contribute to higher minimum temperatures.

The urban area of the Salt Lake Valley has grown considerably over the past few decades, with the population of Salt Lake County increasing from 462,000 in 1970 to 1,049,000 in 2011. This has been accompanied by considerable expansion of development into the southern and western Salt Lake Valley.

Source: Lowry (2010)

The Salt Lake City Airport lies at the edge of the urban area, so it probably isn't located where the urban heat island is the strongest. That being said, traditionally at the time of minimum temperatures, the low level flow is southeasterly and from the urban area to the airport. Thus, some of the increase in minimum temperature and decrease in DTR over the past few decades is probably related to urbanization. The influence on maximum temperature (in the summer) is less clear.

3. Airport and instrumentation changes. Changes in the local characteristics of an observing site can be extremely important, as discussed in this recent post on Cliff Mass's blog. Moving a site even a short distance, altering the landscape or development around it, or changing the instrumentation can result in major changes in the temperature statistics. The Salt Lake City temperature time series doesn't seen to show any major jumps (up or down) in temperature, but that doesn't mean there isn't a fly embedded in the ointment. I've never dug into the records enough to address this concern, but it is an issue worthy of investigation.

4. Wildcards. There are other factors too numerous to discuss, and all of these factors can interact in unusual ways. For instance, a small increase in incoming long-wave radiation at night can sometimes have a big impact on minimum temperature if it weakens the stability of the boundary layer just enough to allow the turbulence to mix the surface layer. Sometimes a small change in the surface energy balance has a big impact on temperatures, other times not. Much work is needed to understand these effects.

In summary, average summertime maximum and minimum temperatures have increased in recent decades at the Salt Lake Airport. Minimum temperatures have increased more than maximum temperatures, resulting in a decrease in the DTR. It is likely that these trends reflect global warming and urbanization (with climate variability contributing to variations on shorter time scales), with their relative contribution and possible role of changes in land surface and instrumentation at the airport still unexplored. I'd say there's a good research topic here, but I also know that teasing out all these interactions is incredibly challenging.

Tuesday, July 23, 2013

As discussed in the previous post, there's a pretty good chance that the average temperature for this June and July will be the highest observed since 1948 (when I began my analysis) and very possible since the beginning of records in the Salt Lake Valley in 1874 (at least that's as far back as the National Weather Service goes).

The average temperature is typically calculated by averaging the maximum and minimum temperature rather than calculating an average based on hourly observations (although not a necessity currently at the Salt Lake Airport. Such an average, however, obscures the dramatic difference in the trends of maximum and minimum summertime temperatures over the past few decades.

The graph below presents the average summertime maximum temperature at the Salt Lake City Airport each year since 1948. There are large ups and downs from year to year, but a very gradual upward trend during the period. The average maximum summertime temperature during 1948–1957, the first decade of this record, was 87.4ºF. Over the past decade (2003–2012), however, it was 89.6ºF. An increase of just over 2ºF.

Source: http://xmacis.rcc-acis.org

In contrast, the graph below presents the average summertime minimum temperature at the Salt Lake City Airport each year since 1948. One also sees large ups and down from year to year (note that these roughly correlate with the ups and downs in maximum temperature), but the trend is larger and much more apparent. The average minimum summertime temperature during 1948–1957 was 57.9ºF. Over the past decade (2003–2012), however, it was 63.0ºF, an increase of about 5ºF, roughly 2.5 times the rate of the maximum temperature increase.

This is not an uncommon finding. Over the past few decades, many stations have seen a larger increase in minimum temperature than maximum temperature, which reduces what meteorologists call the diurnal (daily) temperature range or DTR.

In a future post, we will talk about possible causes for these trends at the Salt Lake City Airport, but for today I merely want to discuss why the maximum and minimum temperature trends might differ.

The layer of the atmosphere that is in contact with the ground and through which air is mixed is called the boundary layer. In the afternoon during the summer, the boundary layer over Salt Lake City is typically quite deep, often extending more than 2000 meters (6000 feet) above the valley floor. Eddies and turbulence distribute energy and pollution through the boundary layer and keep it well mixed (the updrafts favored by paragliders are a result of larger eddies in the boundary layer). Think of a pot of water in full boil.

In contrast, at night and during the early morning when minimum temperatures are typically observed, the boundary layer over Salt Lake City is typically quite shallow, perhaps 100 m (300 ft) or less. There's usually an inversion present near the surface (unlike our wintertime inversions, summertime nighttime inversions burn off quickly when the sun comes up). Eddies and turbulence are quite weak and only mix air through a shallow layer.

As a result, the nighttime boundary layer is very sensitive to any change in the surface energy balance. If you provide an input of energy during the night, it is distributed through a shallow layer. In contrast, the same energy input would be distributed through a very deep layer in the afternoon. As a result, the same energy input in the morning typically results in a larger temperature increase than during the afternoon.

Chances are you've observed this first hand. What happens when the sun comes up? The temperature rises very rapidly. In contrast, in the afternoon, the temperature rise is typically very slow, despite the fact that the incoming solar energy is much larger. This is because during the morning, the small amount of energy provided by the low-angle sun is distributed through a shallow layer, so it gives you a big temperature increase, whereas in the afternoon, the large amount of energy provided by the high-angle sun is distributed through a deep layer, so you get less bang for the buck (there are some other factors at play, but we won't worry about it for the sake of this demonstration).

The bottom line is that the minimum temperature is more sensitive to a change in the surface energy balance. In the near future we will explore some of the factors that might be causing the warming trend at the Salt Lake City airport, including the decreasing diurnal temperature range.

Monday, July 22, 2013

With meteorological summer (June, July, and August) a bit more than half over, it's worth putting our temperatures thus far into perspective. As you might guess, they are at or beyond the outer edge of what has been observed in the instrumented record.

I'm going to concentrate on data from the Salt Lake City airport since 1948, which is the beginning of records maintained by the National Climatic Data Center for the Salt Lake City Forecast Office. The National Weather Service has data for Salt Lake City back to 1874, but I suspect that is a compendium of data from several locations.

As shown below, this June was the second warmest (in terms of monthly average temperature) since 1948, topped only by 1988 (NWS records put this June as the 3rd warmest since 1874).

Source: http://xmacis.rcc-acis.org

Thus far, this July is the warmest since 1948, topping by just a hair 2007 and 2003.

Source: http://xmacis.rcc-acis.org

Putting those together, if we can maintain our July average temperature at or above their current levels, this will be the warmest June–July period since 1948.

Source: http://xmacis.rcc-acis.org

Looking at the models, I think we have a pretty good chance of that happening. There's some monsoon convection possible at times, and a weak trough that might cool us a touch in a few days, but that's about it. Keep your fingers crossed. If we must suffer, it should be to set records.

Sunday, July 21, 2013

When it is hot, it is the maximum temperatures that tend to get our attention, but the minimum temperatures are also important for health and comfort. When afternoons are hot and uncomfortable, a good cool off overnight or from a thunderstorm can help cool off the house or apartment and make living more bearable.

Unfortunately, the current heat wave has featured not only high maximum temperatures, but also high minimum temperatures. This can be seen in the National Weather Service graphs below, which show bars of the observed temperature range (minimum to maximum temperatures) each day in June and July compared to the average (green color fill) and records (top and bottom edge of red and blue denote record maximums and minimums, respectively).

Source: NWS

There has not been a below average minimum temperature at the Salt Lake City airport since June 23rd. The closest was on July 14, when we had a minimum temperature of 66ºF, compared to an average for the day of 65ºF. The streak will reach a full month on Tuesday.

Wednesday, July 17, 2013

Doping or no doping, the Tour de France is one of the most grueling and punishing sporting events in the world. Today's time trial is currently underway as I write this, but it is likely that Chris Froome will have a multi-minute lead heading into tomorrow's stage, which looks to be a real sufferfest. The riders will navigate 172.5 km and six categorized climbs, including two "hors catégorie" (beyond category) climbs up the punishing Alpe D'Huez.

Source: www.letour.fr

Froome has looked unconquerable thus far, but as the saying goes, it ain't over until it's over. A bad day or a crash can doom anyone, and there are always wildcards in cycling. Tomorrow's wildcard could be the weather. The Alps are presently under the influence of a weak trough, and the models call for showers and thunderstorms to develop over the region during tomorrow's stage.

Wet, slick roads could make already harrowing descents even worse. Tomorrow's stage was already must see TV, but it will be very interesting how things play out if the weather turns nasty.

Tuesday, July 16, 2013

Yesterday's high of 100ºF at the Salt Lake City International Airport gives us day 10 this summer of triple digits. Yup, double digit triple digits. We're now nearly half way through July and are running 6.2ºF above the average temperature for the month. Oh the humanity!

Forecasts for the next 7 days show very little change in the overall large-scale circulation with a long-wave ridge dominating the pattern over the western US. Below is the forecast for 0000 UTC Sunday (6 PM MDT Saturday) showing the ridge in firm control.

Source: NOAA/ESRL/PSD

Subtle variations on smaller scales will give us some weather variability during this period in terms of the coverage and intensity of monsoonal convection, but that's about it. These are the dog days of summer. Personally, I'm hoping for as many highs as possible at or above 100. The high 90s are such a waste.

Monday, July 15, 2013

During the dog days of summer, it's critical to keep your drinks cold. When the dewpoint is high, such as during monsoon surges, you either drink fast or use a beer koozy, as demonstrated above. Here's why.

During situations with high dew points, condensation will form on a cold beer (or soft drink) can or bottle. The rate at which your beer or soft drink warms is thus affected not only by the direct flow of heat from the air to the bottle, but also condensational warming. This warming is the result of a net flow of water molecules from vapor to liquid, which releases what meteorologists call latent heat (the converse of this is evaporative cooling, which you have certainly experienced on dry, windy days in Utah).

How important is condensational warming? This all important issue was recently explored by University of Washington Atmospheric Scientists Dale Durran and Dargan Frierson in an article published in Physics Today(Dale is a former Professor of Meteorology at the University of Utah).

"At 35 °C and a relative humidity greater than 60%, the temperature rise due to latent heating exceeds that due to heat transfer from dry air: Latent heating is the dominant factor warming your cold beer." - Durran and Frierson (2013)

In US units, that equates to a temperature of 95ªF and a dewpoint of about 78ºF, far steamier than we see in Utah [the warming is measured for a 5 minute period, details here, but it may be paywalled for those without a Physics Today license.]. If we picked something like 95ºF with a relative humidity of 35% and a dewpoint of 63ºF, closer to the steamiest weather we get in Utah, their Fig. 1b suggests perhaps 20–25% of the initial warming of your beer is due to condensation.

Beer koozies provide insulation that slows the flow of heat from the air to the bottle, but they also prevent condensation. Note in the photo above that there is no condensation on the koozy. This can be especially important in hot, high humidity environments where the warming of your beer or soft drink is strongly influenced by condensation. In Utah, the condensational warming effect is often negligible (when it is dry and you don't see "sweat" on the container holding your drink), but it can be an added warming factor during monsoon surges and other periods with elevated dewpoints.

Friday, July 12, 2013

It's been a while since we took a look at the large-scale atmospheric circulation, but it seems appropriate today.

Last night, moisture moved into northern Utah, giving us some clouds and thunderstorms that were greatly appreciated after a 104ºF maximum on Wednesday. Clouds linger this morning and we will see more showers and thunderstorms across the state today. The National Weather Service has issued a flash flood watch from noon today through this evening for a good portion of central and southern Utah.

Source: NWS

The pattern producing this weather is commonly seen over the southwest United States during the summer monsoon. As shown in yesterday morning's 1200 UTC analysis, we are under the influence of a strong upper-level ridge that is centered over the southwest US with easterlies across northern Mexico and southern Arizona turning anticyclonically and streaming northward into Utah. This flow is confluent with a drier airstream moving into northern California and northwest Nevada from the west and southwest.

This is a very common occurrence during the monsoon, and the confluent flow pattern often results in dramatic contrasts in weather across the western United States. It enables moist air originating over the Gulf of Mexico and environs to penetrate into the southwest US and the surge northward into Utah and the interior west (the Gulf of California can also be a moisture source) while it remains drier further west. Note in the loop below the strong contrast in integrated precipitable water (a measure of the total water vapor content of the atmosphere) across Nevada (color contours with warmer colors indicating higher values).

Fluctuations in the strength and position of the western US ridge and the Pacific trough, as well as smaller-scale disturbances moving through these features, modulate the monsoon system. As a result, the intensity of the monsoon varies from year to year and, during individual seasons, there are waxes and wanes in monsoon moisture and convection known as monsoon surges and breaks, respectively. We are currently under the influence of a surge, which was strongly influenced by a shortwave trough moving into the western US from the eastern Pacific and additional shortwave troughs moving anticyclonically around the upper-level ridge as seen in the tropopause and 500-mb height analyses below (top two panels).

Being at the northern periphery of the monsoon influence, such surges frequently play an important role in the development of precipitation over northern Utah during the summer.

Addendum @ 12:30 PM 12 July:

Bob Maddox of the Madweather blog sent me a note illustrating that moisture sources from west of the Continental Divide, including the Gulf of California and the remnants of Hurricane Erick likely contributed to the moisture surge. Below are some trajectories going back about 4.5 days illustrating this. Thanks Bob!

Thursday, July 11, 2013

After yesterday's hot and smoky post where I discussed the impact of smoke on air quality, the wind shifted just enough to clear us out some, with an accompanying drop in PM2.5 (note the decline in the brown line just after noon on 7-10).

In addition, although the ozone concentrations were higher than the were the previous day at noon, with the decrease in smoke, they dropped just a bit during the afternoon, resulting in a lower peak (compare blue line above on 7-9 and 7-10). This may be evidence that the hydrocarbons accompanying the smoke were contributing to higher ozone levels.

Unlike our wintertime PM2.5 pollution where we have met the enemy and it is us, the above illustrates that there are some regional-scale factors that contribute to elevated PM2.5 and ozone in the summer. We are still our own worst enemy, but emissions from fires and other regional sources related to fossil fuel combustion play a role.

BTW: Yesterday's high at the Salt Lake Airport reached 104º, matching the record for the day. It was also our 9th triple digit day of the year (average is 5). So far we are running 7.6ºF above average for July.

Wednesday, July 10, 2013

Temperatures at the Salt Lake City Airport have reached the triple digits (100ºF as of 2 PM) and smoke has settled in over northern Utah. The view of the Salt Lake Valley from Snowbird is not as disturbing as it is during wintertime inversions, but it isn't pristine either.

Source: Snowbird

The National Weather Service produced a nice graphic yesterday showing some of the primary smoke sources. Yup, welcome to fire season in the western U.S.

Smoke has a very measurable impact on air quality and contributes to both an increase in particulate matter and ozone (click here for more discussion).

Trend charts from the Utah Division of Air Quality show a gradual increase in PM2.5 over the past two days. In addition, yesterday's ozone was the highest in several days and thus far today we are running a bit higher.

Source: Utah Division of Air Quality

Some of the trend in ozone could be related to the increase in daily mean temperature, but I suspect the big jump that occurred from July 8 to July 9 was partly related to the advection of smoke into the valley. Others with a better understanding of photochemistry can perhaps comment and provide their perspectives.

Tuesday, July 9, 2013

There's been some fairly exciting weather in the Salt Lake Valley the past several days, beginning with the 4th of July thunderstorms, which continued overnight leading to the closure of Little Cottonwood Canyon, and continuing through yesterday afternoon when we had another set of rumblers drop up to 0.43 inches of rain on portions of the valley.

This period of thunderstorm activity is, however, slowly dwindling. Observations of integrated precipitable water from the Salt Lake City airport show a gradual decline over the past two days, indicating a drying of the atmosphere that will lead to reduced thunderstorm activity the next couple of days.

Integrated precipitable water is the depth of water you would have if you condensed out all the water vapor in the atmospheric column above you. As can be seen in the plot above, we were running near or above 2.5 cm (1 inch) over during the past several days, with some higher peaks (the peaks are likely associated with thunderstorms or their outflows, which typically have higher water vapor contents). This morning, we are down to about 1.8 cm (0.75 inches), and that loss of moisture makes it more difficult to develop thunderstorms. Thus, there is a slight chance of thunderstorms today, mainly over the mountains.

Monday, July 8, 2013

Salt Lakers have been on a wild ride this year. Remember how cold, snow covered, and polluted it was in the Salt Lake Vally during January? During the last week of June, didn't that seem like a distant memory? Here's a look back at the past six months in terms of mean temperature and departure from average at the Salt Lake Airport, with blue below and red above average:

January: 19.4ºF (10.1ºF below average)February: 28.6ºF (5.6ºF below average)March: 44.6ºF (1.0ºF above average)April: 49.4ºF (1.2 ºF below average)May: 62.6ºF (2.8ºF above average)June: 75.3ºF (5.6ºF above average)
That is one heck of a rocket ride from extreme cold to extreme heat. As far as I'm concerned, what we experienced this year is a worst-case scenario — very cold in January to very hot in June. If we could switch that around, life would be far more enjoyable. A mild January is still just fine for skiing, and the colder it is in June (and July), the better. Shorten summer as much as possible.

January really gets my attention as it is one of the most anomalous months that I can recall in Salt Lake City. In fact, it was the coldest January in Salt Lake since 1949.

Source: NCDC

The NWS lists June as the 3rd warmest. Thus, its a pretty good guess that this years January to June transition was the largest and nastiest since 1949 (that January was so unbelievably cold that I suspect it will end up with a bigger January to June transition). We should be able to check this out in a couple of days when NCDC releases the climate numbers for June.

Saturday, July 6, 2013

A trip to southeast Alaska should be a high priority for any Wasatch Weather Weenie. I'm presenting some commentary and photos from our recent trip below, perhaps enough to induce boredom, but if your synapses aren't stimulated along the inland passage, you lack creativity and imagination. Further, as unearthedcomics.com/Sara Zimmerman shows below, scientists view vacations in a somewhat unique fashion.

Source: unearthedcomics.com/Sara Zimmerman

We begin in Juneau, where one is immediately blown away by the Mendenhall Glacier (commentary on the image below courtesy of my son).

The common refrain we heard from locals is that they can't believe how much the glacier has receded in their lifetimes. If this retreat were isolated, it might be an indicator of local climate variability, but it is consistent with what most (but not all) glaciers are doing around the world (there are a couple of glaciers in Alaska advancing, including the Taku near Juneau). The visitor's center provides some educational displays discussing global warming, but unfortunately they totally whiff in regards to scientific accuracy. How would you answer this question (hint: as in the Matthew Broderick film War Games, the only way to win is not to play)?

In terms of the diversity of potential natural disasters, Juneau has to be near the top of the list. Earthquakes, tsunamis, landslides, seiches, downslope winds, avalanches, you name it. With no provocation, my son looked at Mount Juneau and commented that it was one of the scariest places he ever saw. He then sent this photo to some friends. If you are interested, a history of Juneau avalanches is available here.

A major change that I have noticed since first visiting southeast Alaska about 20 years ago is that weather forecasting is no longer viewed as a joke. Alaskans recognize that although forecasts remain imperfect, they have advanced to the point that they should be taken seriously. Efforts by the National Weather Service forecast office in Juneau were front page news while we were there.

Of course, Juneau is home to some great and adventurous skiers. Eagle Crest (top photo below) is the home mountain of Olympic Silver Medalist Hillary Lindh. An approach to the ski area offers the opportunity to worship at a ski shrine. Remarkably, the skis at the shrine are still mounted, so a return visit might be needed to do some demoing.

Alaska is noted for highly variable weather, so it is important to hit the T-shirt shops to catch the latest forecast.

Indeed, given that it rains over 150 inches a year in portions of southeast Alaska, it is quite fortunate that we enjoyed beautiful sunsets in both Sitka and Ketchikan.

Lucky? Perhaps, but I've been to southeast Alaska when it's blowin' and rainin' and you feel like you are living in a permanent carwash, so I've paid my dues.

Wednesday, July 3, 2013

It has been quite a run of triple digit temperatures in Salt Lake City. Here's a summary thus far:

June 27: 101

June 28: 105 (record for June)

June 29: 105 (ditto)

June 30: 103 (record for day, longest consecutive string of triple digit days in June, June finishes with no measurable precipitation))

July 1: 104 (record for day)

July 2: 102

This streak is, however, in jeopardy. A short-wave trough is expected to sag into the Pacific Northwest by tomorrow and the ridge weakens just enough to cool us off a touch and increase the chance of thunderstorm.

I think we'll eke out one more triple-digit day today, but the end of the streak could come tomorrow. The high 90s never looked so good.

Tuesday, July 2, 2013

This is a guest blog from Matt Jeglum, a PhD student at the University of Utah. Matt spent 5 years fighting wildland fires.

By now most of you have probably heard about the 19 fatalities that occurred on the Yarnell Hill Fire, a wildfire about 90 miles northwest of Phoenix, AZ that has also burned at least 50 homes in the small town of Yarnell, AZ. The 19 firefighters killed constituted nearly an entire crew and makes the Yarnell Hill Fire the most deadly wildfire in the US since the Griffith Park Fire killed 25 firefighters in 1933. Many thanks go out to those firefighters who continue to risk their lives to save lives and property.

With each major fatality wildfire, extensive investigations have led to increasing safety though improved equipment and tactics. Some of these investigations, including the investigation of the 1990 Dude Fire in Arizona, have included improvements to the way that fire weather information is forecasted and disseminated. Today every firefighter undergoes training to learn how weather and fire interact. Incident commanders can request a Spot Weather Forecast from the National Weather Service that is specific to the location of their fire. Large wildfires may even employ an on-site Incident Meteorologist (from the National Weather Service) along with several dedicated surface weather stations providing real time weather data across the fire area.

Weather plays a major role in firefighters' ability to control a fire, and the Yarnell Hill Fire was no exception. Two publicly available weather stations exist near the Yarnell Hill Fire, and the time series for one of them, Stanton, is shown below.

Temperature and relative humidity at Stanton from Mesowest.com

Wind speed and direction at Stanton from Mesowest.com

The above plots show that on Sunday afternoon the fire area was subject to temperatures near 100 F, relative humidities in the teens, and wind from the southwest with gusts into to 20's. This caused the fire to move to the northeast at about 1/2 mile per hour according to some sources. From the limited information currently available, the Granite Mountain Hotshots were working on the south side of the fire on Sunday afternoon and probably expected the fire to keep moving to the northeast. However, as the afternoon progressed widespread thunderstorms popped up across the northern half of Arizona, moving in a southwesterly direction.

Looking at the above image from the Flagstaff, AZ weather radar one can see these thunderstorms along with numerous outflow boundaries.

Outflow boundaries frequently accompany thunderstorms in the Southwest US and can be diagnosed by looking at weather balloon soundings of the atmosphere. The above sounding was taken at 6 pm Sunday in Flagstaff, AZ. The line on the left is dew point and the one on the right is temperature. The further apart these lines are, the dryer the air is at that altitude. You can see that the moisture supporting the thunderstorms is in the mid-levels of the atmosphere, while the lowers levels (below 550 mb) are very dry. In Arizona on Sunday, precipitation falling from the thunderstorms partially or completely evaporated in this dry air and in doing so cooled the air. This cool air, being more dense than the surrounding air, sank rapidly until hitting the ground and fanning out. After hitting the ground the cool air can travel many miles horizontally, producing dramatic changes in wind speed and direction as it passes by a given point. This is the critical aspect of the weather that led to the tragedy at Yarnell Hill.

Wind vectors at Stanton from Mesowest.com. Vectors point in the direction of wind flow. Down means a northerly wind.

Looking back at the Stanton station, between 5 and 6 pm (1700 and 1800 on the above plot) the outflow boundary from the thunderstorm to the northeast of the fire hits the station. It probably hit the fire just minutes before. The wind switches to northeasterly and the gust speed doubles to nearly 45 mph, with sustained winds near 25 mph. A slight drop in temperature and rise of relative humidity is seen as the boundary passes, consistent with an outflow, but temperatures stay in the mid-90's with humidities still in the teens. This is a perfect storm. With the strong, northerly wind pushing it, the fire completely switches direction and starts burning furiously toward the Granite Mountain Hotshot crew, who are on the south side of the fire at this time. There was probably very little time to react.

As a former forecaster and former firefighter, I can appreciate the issues faced by both sides when it comes to forecasting, disseminating, and using weather information to fight fires. Conditions like this were certainly foreseeable the morning before the accident. However, predicting where thunderstorms will form and where outflow boundaries will go, if there are any, is extremely difficult outside of a realm of forecasting called nowcasting. Nowcasting involves making forecasts on the order of hours to minutes based mainly on extrapolating forward the currently observed conditions. It is very useful for weather conditions such as those experienced on the Yarnell Hill Fire, but is very labor-intensive and impractical over large areas.

There are no doubt many factors a play in causing a tragedy like this. Even the influence of the weather still remains to be fully understood. Hopefully the Yarnell Hill Fire will provide an opportunity to make firefighters safer as they protect lives and homes from wildfires.

About Us

The Wasatch Weather Weenies discuss the weather and climate of the Wasatch Front and Mountains, western United States, and beyond.

Participants include aspiring and old-school atmospheric scientists, weather enthusiasts, powder snobs, and poor souls enrolled in classes taught by University of Utah Atmospheric Sciences Professor Jim Steenburgh. Many posts feature content or insights enabled by the support of the National Science Foundation, Office of Naval Research, and the NOAA/National Weather Service.

Comments should be substantive, weather and climate related, and approved for all audiences.

Make a Donation

If you enjoy this blog and want to help transform weather and climate prediction and education in mountainous regions, click here and make a donation to the Mountain Meteorology Fund at the University of Utah. From advancing powder prediction to preparing for a warming world, your donations make a difference.